Gas lasers are well known for their wide range of applicability due to their great variety of output parameters. For example, gas discharge lasers are capable of producing relatively large amounts of optical power. This is especially true if the gas discharge lasers are operated in a pulse mode having pulse repetition frequencies of up to 1 kilohertz or more.
To operate a gas discharge laser in a pulsed mode, it is necessary that the laser gas be ionized before each pulse of optical energy is produced. After a pulse of optical energy has been produced, it is necessary for the conditions in the discharge region to return to an equilibrium state such that the uniform medium conditions for the next optical pulse will be substantially the same as those for producing the preceding optical pulse. One way to accomplish this restoration of equilibrium conditions is to cause the laser gas to flow through the discharge region at a speed such that the disturbed conditions (including contaminating by-products of the previous electrical discharge) are swept out of the discharge region.
Transverse laser gas velocities of hundreds of meters per second or more can be required to permit successful operation of a pulsed gas discharge laser system at pulse repetition frequencies of 1 kilohertz or more. This, in turn, can require the consumption of significant amounts of power to operate the rotating motors and other components that cause the laser gas to flow transversely.
The energy which is deposited in the laser gas during each pulse of a repetitively pulsed laser is typically many times the energy that is emitted as laser light during that pulse. Typical laser efficiencies vary from less than one percent to as high as 15 to 20 percent. The electrical energy for exciting or pumping the laser gas for a pulsed laser must be added to the gas very rapidly, i.e., tens of nanoseconds to tens of microseconds, to have relatively high efficiencies. The energy addition is essentially instantaneous relative to the gas motion, and heats the gas at essentially constant volume. This raises both the gas pressure and temperature in the discharge region, in direct proportion to the energy added, typically to 1.3 or more times the initial energy, temperature, and pressure for a discharge laser.
The laser pulse is typically over in a few nanoseconds to a few microseconds, depending on the laser type. The residual hot, high pressure gas generally expands very rapidly, forming shock waves that propagate upstream and downstream of the energy addition region, disrupting the gas flow and causing undesirable flow disturbances. The expansion process returns the pressure of the gas in the energy addition region to its initial pressure in an unsteady manner, after which it would be very difficult to use the hot residue gas to self-generate flow. This hot, low pressure residue must then be purged from the energy addition region. The expansion energy is initially available within the high pressure gas in the laser cavity immediately after a discharge or pumping chemical reaction pulse, and can be used to generate gas motion during each pulse and on a continual basis.
In the past, this energy and the unsteady gas motions and pressure disturbances that it generates have been treated as undesirable for laser operation. The flow work or energy required to purge gas from the energy addition region per pulse, W.sub.f, depends on the pressure loss in the flow loop, .alpha.p.sub.1, which in turn depends on the cavity dynamic head, q.sub.c, which is a function of the gas density, .rho., the cavity flow velocity, u.sub.c, and the flow loop configuration which causes a total loss of some multiple, N, of the cavity dynamic head. The relationship of these factors is expressed by the following two formulas: EQU q.sub.c =0.5.rho.u.sub.c.sup.2 EQU .alpha.p.sub.1 =Nq.sub.c
The work per pulse W.sub.f to purge the hot residue from the laser cavity also depends on the volume of this region, V.sub.1, and the distance that it has to be purged from its initial location relative to its initial width, W, normally referred to as the flush factor, F. The flow work per pulse is EQU W.sub.f =.alpha.p.sub.1 FV.sub.1
For a typical discharge laser with a laser cavity region of 4 cm flow width, 4 cm discharge height, and optical length of 100 cm, pulse repetition frequency of 50 Hz, a flush factor of 3.0, a molecular weight of approximately 20, and flow loop loss of 4.0 times the cavity dynamic head, the flow work per pulse is approximately 2.8.times.10.sup.6 dyne-cm.
The energy that is not emitted as laser light is thermalized within the gas in the laser cavity. This process takes place very fast relative to typical flow times, and fast compared to acoustic transit times across the laser cavity. The process is thus essentially instantaneous relative to gas motions, and results in what is generally referred to in thermodynamics as constant volume heating of the laser gas within the laser cavity. This constant volume heating process leaves the gas within the laser cavity at a high pressure and temperature with respect to the gas in the rest of the flow loop, on the order of 1.3 times the initial temperature and pressure for a typical discharge laser or 3.0 to 10.0 times the initial conditions for a pulsed chemical laser.
In conventional laser flow loops this discontinuity in pressure and temperature causes shock and expansion waves to form immediately after the laser pulse, and generates unsteady flows and pressure disturbances which must be suppressed while the hot residue is purged from the laser cavity prior to the next laser pulse occurring. However, the hot, high pressure gas in the laser cavity can be used to generate a continual flow of gas throughout the flow loop through the application of components that direct unsteady flows in a preferred direction and thus create the needed purge flow. The expansion power available in the hot, high pressure laser residue is sufficient to drive flow through the high pressure loss components needed to cool and homogenize the gas in a very compact flow loop. The expansion work available immediately after the energy addition depends on the initial pressure, p.sub.1, the energy addition volume, V.sub.1, the specific heat ratio of the gas, .gamma., and the heated gas pressure, p.sub.2, by the equation ##EQU1##
This is typically one hundred times, or more, than the flow work, W.sub.f, per pulse, or flow power, that is required to circulate flow within a laser flow loop. In general, the ratio of the expansion work available per pulse to the flow work required to purge the hot residue is: ##EQU2##
The available energy is many times the energy per pulse needed to drive the gas flow required to purge the laser cavity for many pulsed lasers. Thus, the conversion of power available in the hot, high pressure residue into flow power can be inefficient and still generate adequate purge flow, or it can be made efficient and provide sufficient flow power to drive flow through relatively high resistance components or a high pressure loss flow loop.
It is therefore desirable to produce a pulsed laser system having an ultracompact flow loop with no rotating components. Through appropiate design of the laser flow loop or other flow loop, the energy added by each pulse of the laser operation can be used to establish an average flow in a desired direction within an appropriately designed flow loop without having to incorporate fans, blowers, compressors or other conventional circulating devices.
Generally, very compact flow loops have high pressure losses due to the small flow passages and relatively high flow velocities, the pressure loss being proportional to velocity squared. Thus, the expansion work available in the hot residue immediately after a pulse can provide a means for self-generating the flow power needed to circulate gas within a very compact flow loop.
In summary, the waste energy that is deposited in the laser gas contained within the laser cavity of a repetitively pulsed gas laser can generate a continual flow of gas within a laser flow loop, and thus eliminate the need for a circulating fan, blower, or other rotating equipment. This gas flow can be used to purge the laser cavity and restore a uniform medium at the initial conditions for the next laser pulse. The relatively large amount of energy that is thermalized in the gas due to the inefficiency of typical discharge or chemically pumped gas lasers can provide a relatively large amount of pumping power to the gas. The potential work available in the waste gas immediately after the pulse makes it possible to construct a self-activating heat engine around the pulsed energy addition region to circulate the gas within the flow loop. This available flow work can provide a means for building very compact flow loops for repetitively pulsed gas lasers, as discussed above, and other pulsed devices which require purge flows.
While an ultracompact flow loop offers the advantages mentioned above, it also offers the advantage of significantly more compact design and significantly less mass. Conventional flow loops have a size which may be related to the energy addition or discharge height, H, and typically have a diameter that is between 10 and 50 times this height. The diameter of the self-pumping flow loop of the invention may be as small as two to three times the discharge height. This represents a very large reduction in flow loop volume and a correspondingly large reduction in weight.
In an alternative configuration of the invention, very small cross section ducts between flow components and small flow cross section components such as heat exchangers and thermalizers are used. The small cross section of such ducts will generally mean a large pressure loss due to high flow velocity. The high flow velocity, in turn, conventionally requires prohibitively large fans or blowers, large drive motors, and a significant penalty in laser efficiency. Thus, it is desirable to have an ultracompact flow loop which can self-generate large gas flows and flow power by utilizing self-actuating unsteady flow valves or passive, one-way unsteady flow components.